ACTION OF THE ORBITAL MUSCLES. 591 



movements are accomplished by various combined coordinations 

 of movements of the different muscles. 



The diagram shows the directions towards which the different 

 muscles tend to draw the centre of the cornea from the straight 

 position. 



From this it is obvious that the commonest movements of the 

 eye require the cooperation of different muscles. 



r.iht 



r.ext. r.suj). r.znt 

 r.inf. 



Diagram of the direction of the action of the muscles of the eyeball, 

 which is shown by the dark lines. The axis of the rotation caused by the 

 oblique and upper and lower recti are shown by the dotted lines. The 

 inner and outer recti rotate the ball on its vertical axis, which is cut across. 

 The abbreviated names of the muscles are affixed to the lines. 



In the ordinary movements of the two eyes more than this is 

 necessary. The two eyes must move in the same direction at the 

 same time, now to the right, now to the left, so that while the 

 external rectus moves the right eye to the right side, the internal 

 rectus moves the other eye in the same direction. We say then 

 that the coordination of the movements of the muscles of the 



